Various techniques are known for casting directionally solidified articles such as turbine blades and vanes. In the case of single crystals, a common method involves the use of a starter zone at the bottom of a mold wherein a plurality of columnar grains are formed A "non-linear" or transversely displaced crystal selector (e.g., a helix-shaped passage) connects the starter zone to the article cavity, and this selector insures that only one columnar grain grows into the article cavity. Single crystal castings also can be produced using molds which have a vertical "slender projection" at the bottom of the article cavity (i.e., a linear or non-transversely displaced "neck"), or using seed crystals, as described in Bridgman U.S. Pat. No. 1,793,672.
When traditional directionally solidified (columnar-grained polycrystalline) articles are desired, the starter zone communicates directly with the article cavity (no crystal selector or seed crystal is present) as described in Chandley U.S. Pat. No. 3,248,764, VerSnyder U.S. Pat. No. 3,260,505, and Piearcey U.S. Pat. No. 3,494,709.
Directionally solidified articles of either the polycrystalline or single crystal type may be cast in molds which are supported on a chill plate. The temperature gradient during solidification is established in part by selectively controlling the power input to one or more heating coils surrounding the mold. The coils are axially spaced along the vertical axis of the mold, and the mold is heated to a temperature above the pouring temperature of the alloy in order that there will be no nucleation in the mold other than at the bottom of the mold in the location of the chill plate. By control of the heat input and other parameters during the casting operation, a substantially unidirectional thermal gradient can be maintained and the solidification will occur gradually with the single or multiple columnar crystals growing axially within the mold. A general discussion of procedures of this type may be found in Phipps, et al. Pat. No. 3,712,368 and in Tingquist, et al. Pat. No. 3,841,384.
When casting articles which are of irregular design, problems can arise when apertures, small cavities, or the like are insufficiently filled with molten metal as solidification occurs. Upon cooling, shrinkage voids will occur which can result in weakened areas, and rejection of castings. In the case of turbine blades, vanes, and the like, airfoil sections are bounded by roots, shrouds, and "angel wing" portions, and these are casting areas characteristic of regions where shrinkage voids can be found.
It has been particularly found that the production of components such as turbine blades which have a relatively complex configuration can result in shrinkage voids when an attempt is made to produce unidirectionally solidified columnar grained or single crystal castings. More particularly, the extremities of the root and shroud portions of such castings have been characterized by unduly frequent shrinkage defects. These defects are not acceptable for parts which are designed for high performance applications and rejection of such castings result.